Permanent magnet brushless (PMBL) motors can exhibit efficiency and power density suitable for a range of applications. One type of PMBL motor, referred to as a “double-stator PMBL” (DS-PMBL), uses an inner stator, surrounded by a rotor carrying permanent magnets, in turn surrounded by an outer stator. The inner stator and outer stator carry respective stator windings, and these are energized to produce rotating inner and outer magnetic fields. The inner and outer rotating fields each interact with the rotor's permanent magnets and the interactions urge separate, identical direction, torques. The DS-PMBL motor, by that combination of torques urged on the rotor by the inner and outer stator fields, can produce higher torque than a single-stator PMBL.
Among parameters specifying DS-PMBL motors is the number of poles. The number of poles can be significant parameter because increasing the number, subject to various constraints and conditions, can relate to increase in torque density. However, employing conventional techniques of DS-PMBL rotor and PM structure and design, there can be certain significant costs to increasing the number of poles. One of such costs is a phenomenon known as “cogging torque,” which can manifest as a periodically modulated torque on the rotor. Cogging torque can be a greater issue with DS-PMBL motors because such motors possess two air gaps. Another cost, or group of costs, that can arise from the current rate of increase in design, materials and fabrication costs, and costs of necessary over-design, conventional DS-PMBL rotor and PM structures for higher pole count, and resulting costs, for example, fabrication, and materials.